The invention relates to a building board, especially a lightweight building board, having a coating, wherein at least one end face is covered by two superposed layers of sheet material. The invention further relates to a method of coating a building board.
The term building board designates boards, e.g. for the furniture industry, which have a core sandwiched between two cover plates. The core may be formed by a material, e.g. chipboard, that extends over the entire surface. As an alternative, the core may be composed by different materials and may for example comprise a frame of chipboard and a filling of a material that has a lower density. Among others, cardboard in honeycomb-configuration is used as filling. In view of their low weight, such boards are also termed lightweight building boards.
In order to obtain a good and uniform surface finish which may for example serve as a substrate for a high grade varnish, it is known to wrap building boards with a layer of a thin sheet material, e.g. paper or plastic film, in order to coat the same. In case of MDF boards (medium density fibre boards) which are frequently used as cover plates of building boards, the relatively smooth surface of this material assures that a sufficiently smooth finish of the main surfaces (top surface and bottom surface) of the building board is achieved by coating with a thin layer. However, when chipboard is used for the core or the frame of the core, the building boards have a relatively rough end face which cannot be sufficiently smoothened by a single-layer coating.
One possibility to avoid that the roughness of the end faces of the core imprints in the coating consists in filling the depressions in the end faces with hot-melt adhesives that have a high content of filling material, and grinding the end faces after the adhesive has cured and before the coating is applied. However, such adhesives are intricate to handle, which results in high costs for this method.
Another possibility has been disclosed in DE 199 21 189 A1. In the method disclosed in this document, a coating is formed by two layers of paper, a first paper layer covering one of the main surfaces of the building board and one end face, and a second paper layer covering the opposite main surface of the board and also the end face that has already been covered by the first paper layer. In this way, the end face is coated with two superposed layers of paper, whereby a greater amount of roughness can be smoothened out. However, when a particularly high grade surface finish is required, e.g. a mirror-gloss varnish, even this method does not provide sufficiently smooth end faces, at least when thin layers of paper are used. Thin paper layers are preferred to thicker paper layers because of their better processing characteristics and lower costs.